There are clinical and analytical chemistry applications that require precise manipulation of small liquid samples. For example, the analysis of a whole blood specimen may require several manipulations including separation of erythrocytes, leukocytes, and platelets from plasma; dispensing the plasma to one or more reaction volumes; mixing reagents; incubating plasma and reagent mixtures; and performing optical or electrochemical measurement of the treated samples. In addition, advanced protocols may require separating different plasma proteins.
Depending on the overall system requirements, the analyte or liquid movement may be driven by a pressure or displacement source, capillary forces, electroosmotic forces, thermocapillary forces, magnetohydrodynamic forces, centrifugal forces, acoustic energy, or electrophoresis. In many of these applications, the pumps, power supplies, valves, motors, and other hardware needed to implement a complete system are much larger and more expensive than the microfluidic component.
Several technologies have been developed in an effort to minimize sample volume and integrate more system functions within a single device. In one, droplets are immersed in a second dielectric (e.g., water droplets surrounded by a working fluid with a lower dielectric constant) employing a plurality of segmented planar electrodes arranged on top and bottom of a liquid housing.
In another, planar electrodes are used to move a drop of fluid from one electrode to the other. Forces of electrical origin cause the droplet movement. In some cases, depending on the properties of the liquid in the droplet and surrounding working fluid, and the characteristics of the electrode arrangement and excitation frequency, the net effect may be an observable change in contact angle at the tri-phase contact line between a solid, the droplet, and the working fluid. This contact angle change is termed “electrowetting.”
The use of electrical forces has been demonstrated by non-capillary rising of an essentially non-conductive liquid between two metal plates partially immersed in the fluid, one at ground and the other one at a high voltage. As seen in equation (1), the electrical force density on a piece-wise uniform incompressible linear dielectric liquid, fe, is generated by either the presence of a charge density, ρ, driven by an electric field, Ē; or by the action of the gradient of the scalar Ē·Ē (i.e., the square of the electric field magnitude) on a polarizable material with a dielectric constant ∈r relative to the permittivity of free space, ∈0. The first term in (1) is the Coulombic force density and the second term is the Kelvin polarization force density (also known as the dielectrophoretic force density on the liquid).
                              f          e                =                              ρ            ⁢                                                  ⁢                          E              _                                +                                    1              2                        ⁢                                          ɛ                o                            ⁡                              (                                                      ɛ                    r                                    -                  1                                )                                      ⁢                          ∇                              (                                                      E                    _                                    ·                                      E                    _                                                  )                                                                        (        1        )            
For a liquid with spatially uniform properties, the Kelvin polarization force density can only be generated when the geometry of the electrodes establishes an electric field gradient in the liquid. In a conductive liquid with a short dielectric relaxation time compared to the period of the voltage excitation waveform, internal electric fields and electric field gradients are reduced. In the limit of a perfect conductor, the internal field is null. Thus, as the conductivity of the liquid is increased, internal fields are reduced, and charge accumulates at material interface regions. In such cases, Coulombic forces acting on the surface charge at material interfaces are the primary contributors to the electrical force density.
Another technology relies on liquid actuation provided by the Kelvin polarization force (liquid dielectrophoresis). In that case, low to moderate conductivity liquids are handled by modulating the electric field such that the period of the applied voltage oscillations is much smaller than the characteristic relaxation time for the system.
Generally, these technologies require the use of an immiscible working fluid surrounding aqueous droplets (e.g., octyl alcohol and silicon oil, respectively) for best results. Partitioning of chemical constituents from the droplets to the surrounding working fluid is a concern for these two technologies.
In further technologies, co-planar electrode strips covered by a thin dielectric are used as a means for generating a “dielectrophoretic liquid finger” and a string of droplets when the electric field is removed. In these technologies the cross-sectional shape of the finger changed depending on the applied frequency of the field.
Thus, a need still remains for a microfluidic liquid stream configuration system, for biochemical assay analysis, that is capable of dynamically configuring a liquid stream using no moving parts and a minimum of external components. Such system would enable implementing complex biochemical or molecular chemistry analyses in a compact and inexpensive system, making it ideally suitable for point-of-care diagnostics or for home-based diagnostics. Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.